A number of oilfield operations utilize corrosive fluids (e.g., acids) which must be transported to the well site in substantial volumes. Due to the hazardous nature of these fluids, great care must be taken when transporting them over the public highways. Numerous regulations are in effect regarding the transport of such materials.
Typically, corrosive fluids used in servicing oil wells are transported in metal tanks carried on trucks or trailers. These tanks typically have access openings, or manways, formed therein which are surrounded by bolted flanges. These manways are typically closed using metal manway, or access, covers such as the Model 1890 access cover sold by Clay & Bailey. The Clay & Bailey Model 1890 access cover meets the requirements of Department of Transportation (DOT) specification 412 at a test pressure of 36 psi.
The Clay & Bailey Model 1890 access cover includes a base portion having a flange adapted to be bolted to the flange surrounding the manway of a fluid tank. An access cover is connected to the base portion of the Clay & Bailey apparatus by a hinge. A plurality of cam locking arms fold over the metal cover when it is in a closed position, thus locking the metal cover in place. The inner surface of the Clay & Bailey cover may be lined in order to protect the cover from corrosion. However, this corrosion-resistant liner is susceptible to being torn away from the cover or otherwise damaged. Moreover, the Model 1890 cover includes numerous intricately shaped exterior metal components, such as cams, latches, pins and the like, which are susceptible to corrosion.
Non-metallic containers, such as those sold by Poly Processing Company of Monroe, Louisiana under the designation TRANSTORE TANKS SERIES II, have also been used for storing and transporting corrosive fluids. The tanks sold by Poly Processing Company are manufactured from high density polyethylene and have integrally molded access openings which include female threads for threadingly receiving male threaded cover plugs. These male threaded cover plugs have hammer lugs formed therein.
Non-metallic tanks for storing and transporting corrosive fluids have also been sold by Bonar Plastics under the designation POLY PAYLOADER.RTM.. Each of these containers has an integrally molded neck opening with external male threads defined thereon. A molded non-metallic cap screws onto the threaded neck to close the neck opening.
Neither the TRANSTORE TANKS SERIES II tanks nor the POLY PAYLOADER.RTM. tanks just described meet the requirements of DOT specification 412 at a test pressure of 36 psi. Rather, these tanks meet only the requirements of DOT specification 57 at a test pressure of 15 psi.
Providing a metallic tank with non-metallic, i.e., polymer, liners are known within the art. However, polymer lined tanks can be prone to developing cracks within the liner to hatch-flange interface, thereby exposing the metallic wall of the tank to the corrosive liquid contained within the liner. Furthermore, this cracking problem is aggravated when multiple openings, each having a respective flange for receiving a hatch or a preselected component, are provided within a single tank. Thus, it would be beneficial to the art to be able to reduce the likelihood of such stress-induced cracking from occurring, especially in the case where multiple openings in a single tank are required.
U.S. Pat. No. 5,111,955 discloses a non-metallic hatch designed for use on an acid tank. The non-metallic hatch of U.S. Pat. No. 5,111,955 meets the requirements of DOT 412 at a test pressure of 36 psi. The non-metallic hatch includes a base and a lid. The base includes a base flange which is complementary to, and is constructed to be bolted to, an acid tank manway flange. The base also has an opening defined therethrough and includes a male threaded portion which surrounds the opening. The hatch lid has a female threaded portion thereon which is complementary to the male threaded portion of the base so that the lid can be threadedly engaged with the base in order to close the acid tank manway.